Auxin is the chemical messenger in plant shoots that tells the shoot which way to grow: towards light and water, away from the ground, etc. Charles Darwin himself, with his son, Francis, did a series of classic experiments showing that auxin gradients caused shoots to grow in a particular direction.

By running DNA microarrays on Arabidopsis seedlings at different times of day, Harmer and Covington show that almost all steps in the auxin pathway are influenced by the plant’s internal clock. Growth signals are strongest in what should be the late night, or shortly before dawn.

The clock appears to act by ‘gating’ the auxin signal so it is more effective at some times of day than others: at the right time of day, the gate opens and lets the signal through.